Concentration and Temperature Dependency of Regio-and Stereoselectivity in a Photochemical [2+2] Cycloaddition Reaction (the Paterno-Buchi Reaction) : Origin of the Hydroxy-Group Directivity
Concentration and Temperature Dependency of Regio-and Stereoselectivity in a Photochemical [2+2] Cycloaddition Reaction (the Paterno-Buchi Reaction) : Origin of the Hydroxy-Group Directivity
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光化学中区域选择性和立体选择性的浓度和温度依赖性 [2 2] 环加成反应（Paterno-Buchi 反应）：羟基方向性的起源
DOI：
10.1021/ja1088524
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发表时间：
2011
影响因子：
15
通讯作者：
M.
中科院分区：
文献类型：
--
作者：
Yabuno;Y.; Hiraga;Y.; Takagi;R.; Abe;M.
一组光化学氧杂环丁烷形成反应，即，研究了四氢苯并呋喃醇衍生物1a-d与二苯甲酮（BP）的Paterno-Bu-Bu-Chchi（PB）反应，以考察羟基方向性对区域和立体选择性的影响。烯丙醇1a，d的PB反应的选择性在很大程度上取决于烯丙醇的浓度和反应温度。在高浓度烯丙醇存在下，区域选择性随温度的变化与羟基保护的甲基醚1和四氢苯并呋喃（1c）的变化相似。浓度对区域选择性的影响可以通过氢键聚集体来解释，氢键聚集体模拟了1b，c的PB反应期间观察到的选择性。低浓度的1a，d生成的高取代氧杂环丁烷3a，d的羟基顺式选择性大于高浓度的1a，d生成的羟基顺式选择性。3a，d的顺式选择性高于低取代氧杂环丁烷2a，d。顺式构型的氧杂环丁烷的区域选择性高于反式构型的氧杂环丁烷。这些实验结果强烈表明，羟基基团的方向性，诱导的氢键稳定，起着控制PB反应的区域和立体选择性的作用。立体效应也是导致非对映选择性的原因，3d的顺式选择性高于3a。在（U）MP2和（U）DFT理论水平上的计算研究表明，氢键稳定性仅在羰基和烯烃的三重激发态之间的激发复合物（激基复合物）中变得重要，其中发生从烯烃到激发羰基的电荷转移，使羰基氧亲核。没有显着的稳定化能量被发现在中间三重态的双自由基。实验和计算相结合的研究已经澄清了羟基对高度区域和立体选择性的影响的来源，即，激基复合物中氢键稳定性和取代基的空间体积的协同效应。
A set of photochemical oxetane formation reactions, i.e., the Paternò−Büchi (PB) reactions, of tetrahydrobenzofuranol derivatives1a−dwith benzophenone (BP) was investigated to examine poorly understood hydroxy-group directivity on regio- and stereoselectivity. The selectivities of the PB reactions for allylic alcohols1a,dwere found to be largely dependent upon the concentration of the allylic alcohols and the reaction temperature. The temperature-dependent change of the regioselectivity at high concentrations of allylic alcohols was similar to that of the hydroxy-protected methyl ether1band tetrahydrobenzofuran (1c). The effect of concentration on regioselectivity can be explained by the hydrogen-bonded aggregates, which mimic the selectivities observed during the PB reaction of1b,c. The hydroxy-directed cis-selectivity of the higher-substituted oxetane3a,dformed at low concentration of1a,dwas found to be larger than that at the higher concentration of1a,d. The cis-selectivity of3a,dwas found to be higher than that of the lower-substituted oxetane2a,d. The regioselectivity of the cis-configured oxetanes was higher than that of the trans-configured oxetanes. These experimental results strongly suggest that hydroxy-group directivity, induced by hydrogen-bonding stabilization, plays a role in controlling the regio- and stereoselectivity of the PB reactions. The steric effect was also responsible for the diastereoselectivity, as shown by the fact that the cis selectivity in3dwas higher than that in3a. Computational studies at the (U)MP2 and (U)DFT level of theory revealed that hydrogen-bonding stabilization becomes important only in the excited complex (exciplex) between the triplet excited state of carbonyls and alkenes, in which the charge transfer occurs from the alkene to the excited carbonyl to make the carbonyl oxygen nucleophilic. No significant stabilization energy was found in the intermediary triplet state of biradicals. The combined experimental and computational studies have clarified the origin of the poorly understood hydroxy-group effect on a high degree of regio- and stereoselectivity, i.e., the cooperative effect of hydrogen-bonding stabilization in exciplexes and the steric bulk of the substituents.